The present invention relates to a pyroelectric infrared sensor for sensing the infrared rays radiated from a moving body such as the human body.
As an apparatus for detecting the trespass of somebody on a room, a pyroelectric infrared sensor has recently come into wide use. The pyroelectric infrared sensor is provided with a pair of elements for sensing the infrared rays radiated from the human body, and is used for control such as prevention of crimes by utilizing current signals output from the pair of elements in correspondence with the dose of radiation of the infrared rays. A pyroelectric sensor composed of a hybrid IC has recently been widely utilized in the light of reduction in the size of an apparatus and the easy handling property.
FIG. 5 is a sectional view of the structure of this kind of pyroelectric infrared sensor composed of a hybrid IC. In FIG. 5, on the upper side of a substrate 1 of alumina (Al.sub.2 O.sub.3) are provided a pair of elements 2 for outputting currents corresponding to the dose of radiation of infrared rays. In order to enhance the sensitivity of detection of the element-pair, it is necessary to efficiently convert the quantity of heat obtained from the infrared rays into a current signal. For this purpose, in a conventional apparatus, the element-pair 2 is disposed on the substrate 1 with a predetermined space 3 so as to prevent the heat caused by the infrared rays and received by the element-pair 2 from being released through the alumina substrate 1 having a good thermal conduction (the thermal conductivity of this kind of alumina substrate is 0.07 to 0.4 (cal/cm sec.degree.C.) and the element-pair 2 and the substrate 1 are supported and fixed at four points of the respective corners of the element-pair 2 by conductive pastes 4.
On the substrate 1 a resistor 5 and an FET (Field Effect Transistor) 6 are provided, and the current signals output from the element-pair 2 are converted into a low impedance voltage signal defined by V=Q/C according to the impedance transformation by means of the FET 6. The reference numeral 7 represents a terminal pin for supplying a drive power for driving the circuit such as the element-pair 2, the resistor 5 and the FET 6, and taking out the current signal from the element-pair 2. The reference numeral 8 represents a stem for supporting the terminal pin 7.
However, since the conventional pyroelectric infrared sensor uses an alumina plate having a good thermal conduction for the substrate 1, when the shapes such as the diameters of the conductive pastes 4 at the respective points are different from each other, the quantities of heat released toward the substrate 1 through the respective conductive pastes 4 are different depending on the positions. Consequently, the current signals output from the element-pair 2 become out of balance, which is an obstacle to the processing of the current signals. In order to avoid such an obstacle, it is necessary to make the shape such as the diameter and the length of the conductive paste 4 at each position uniform, which is very difficult in the process for manufacturing the sensor.
In the conventional sensor, the current signals output from the element-pair 2 are first converted into a voltage signal by the impedance transformation by means of the FET 6. On the other hand, since the sensitivity of the infrared sensor is influenced both by the thermal time constant .tau..sub.T when the heat due to infrared rays is converted into a current signal and by the electric time constant .tau..sub.E when the FET 6 is operated on/off, the impedance transformation carried out by means of the FET 6 is an obstacle to the enhancement of the sensitivity of detection of the infrared sensor.